Fuel cells can be classified into phosphoric acid fuel cell, alkaline fuel cell, molten carbonate fuel cell, solid oxide fuel cell, polymer electrolyte fuel cell, etc., according to the type of electrolyte that they use. Among them, the polymer electrolyte fuel cell (PEFC), which is capable of low temperature operation and has high output density, has been commercialized as a power source for automobiles and for home cogeneration systems.
Fuel cells are considered as a promising future power source for portable devices such as notebook computers, cell phones and personal digital assistants (PDAs). Because fuel cells do not require charging as secondary batteries do and they can generate electricity by simply feeding a fuel, the inclusion thereof makes portable devices convenient to use.
The PEFC, in particular, is attracting a lot of attention as a promising power source for portable devices because they have a low operating temperature. Among the PEFCs, direct oxidation type fuel cells are considered as the most promising because they can offer an electric energy by directly oxidizing a liquid fuel at room temperature without reforming the liquid fuel into hydrogen. In addition, because direct oxidation type fuel cells do not require a reformer for reforming a fuel into hydrogen, smaller power sources can be readily achieved.
As a fuel for direct oxidation type fuel cells, low molecular weight alcohols or ethers are investigated. Among them, methanol is considered as the most promising because methanol can offer high energy efficiency and a high power output. The fuel cell utilizing methanol as a fuel is called “direct methanol fuel cell” (hereinafter simply referred to as “DMFC”).
The reactions on the anode and the cathode of the DMFC can be expressed by the following reaction formulas (1) and (2). Oxygen serving as an oxidant supplied to the cathode is usually obtained from the air.CH3OH+H2O→CO2+6H++6e−  (1)3/2O2+6H++6e−→3H2O  (2)
A typical structure of DMFC will be described with reference to FIG. 1.
FIG. 1 is a schematic diagram of an example of a polymer electrolyte fuel cell. Direct methanol fuel cells also have a similar structure. An electrolyte membrane 1 is proton conductive. The electrolyte membrane 1 having an anode catalyst layer 2 on one surface thereof and a cathode catalyst layer 3 on the other surface is called a “catalyst coated membrane” (hereinafter simply referred to as “CCM”). A combination of the CCM, an anode water-repellent layer 4, a cathode water-repellent layer 5, an anode gas diffusion layer 6 and a cathode gas diffusion layer 7 is called a “membrane electrode assembly” (hereinafter simply referred to as “MEA”).
In the polymer electrolyte fuel cell, the MEA is sandwiched by an anode-side separator 8a and a cathode-side separator 8b. The anode-side separator 8a has a fuel flow channel 9a for supplying a fuel to the anode. The cathode-side separator 8b has an air flow channel 9b for supplying air to the cathode. These separators and the MEA together form a unit cell for a fuel cell.
A single unit cell generates a voltage of only 1 V or less, and thus it is difficult to drive a device only with the voltage offered by a single unit cell. Accordingly, a plurality of unit cells are usually arranged in series to provide a high voltage. This is called a stack.
The layers included in each MEA are bonded by hot-pressing. However, the interfaces between the MEA and the separators are not bonded. Accordingly, the stack is clamped by applying a pressure in a stacking direction of the MEAs and the separators so as to reduce the contact resistance of the interfaces between the MEA and the separators.
The power generation reaction of fuel cell occurs inside the CCM. In a fuel cell, gas diffusion layers and water-repellent layers are indispensable. Because a fuel cell has gas diffusion layers and water-repellent layers, supplied fuel and supplied air can be diffused evenly into catalyst layers, and products such as water and carbon dioxide can be smoothly transferred to the outside.
The gas diffusion layers are usually formed of a porous material such as carbon paper or carbon cloth. The water-repellent layers comprise, for example, a conductive material such as carbon powders and a water-repellent material such as a fluorocarbon resin. The fluorocarbon resin can be, for example, polytetrafluoroethylene (PTFE).
The water-repellent layers have the following three functions in a fuel cell. One is to allow easy bonding between a gas diffusion layer and a catalyst layer. Secondly, because the water-repellent layers have water repellency, water produced at the cathode can be efficiently transferred to the outside. Thirdly, the projected fibers of carbon paper or carbon cloth serving as a gas diffusion layer can be prevented from penetrating a catalyst layer and an electrolyte membrane. Therefore, an internal short-circuit resulting from the above problems can be prevented.
The water-repellent layers are usually produced in the following procedure. First, a water-repellent paste including carbon black, PTFE, a dispersing medium, a surfactant, etc. is prepared. The water-repellent paste is applied onto a gas diffusion layer using a coating apparatus such as a doctor blade, which is then dried and baked to remove the dispersion medium, the surfactant, etc. Thereby, a water-repellent layer is obtained.
Japanese Laid-Open Patent Publication No. 2004-71508 proposes a coating apparatus having an improved structure for forming a uniformly thin water repellent layer on a gas diffusion layer without-creating cracks.
However, when the interface between the catalyst layer and the water-repellent layer is observed by a scanning electron microscope (SEM), interstices can be found in some parts of the interface. The presence of such interstices can decrease the electron conductivity of the interface between the catalyst layer and the water-repellent layer and increases an ohmic resistance, which may degrade the power generation capability of the fuel cell. Also, water produced by the power generation reaction may accumulate in the interstices of the interface at the cathode. The accumulated water inhibits air to diffuse into the catalyst layer, which degrades the power generation capability of the fuel cell.
The reason why such interstices are created will be described with reference to FIG. 2. FIG. 2 is a schematic cross-sectional diagram of a water-repellent layer 12 produced by a conventional method. FIG. 2 shows a case where the gas diffusion layer 11 is a carbon cloth. In a conventional method, the water-repellent layer 12 is formed by applying a water-repellent paste onto the gas diffusion layer 11, followed by drying. Because the carbon cloth comprises warp and weft threads, intersections of warp and weft threads are included in the carbon cloth, in other words, protrusions and recesses are periodically present in the carbon cloth. If the gas diffusion layer 11 has asperities (i.e., protrusions and recesses) on the surface onto which the water-repellent paste is applied, the surface of water-repellent layer formed on the gas diffusion layer 11 is affected by the asperities of the gas diffusion layer 11. In other words, the surface of the water-repellent layer 12 formed thereon also have the asperities. Even in a water-repellent layer 12 produced using a coating apparatus proposed by Japanese Laid-Open Patent Publication No. 2004-71508, it appears that its surface is affected by the asperities present on the surface of the gas diffusion layer 11. Accordingly, the proposal of Japanese Laid-Open Patent Publication No. 2004-71508 fails to provide a water-repellent layer having a smooth surface. Such asperities depend on the material used for the gas diffusion layer. Although carbon paper and carbon non-woven fabric have a relatively small asperities on the surface thereof, it is still difficult to obtain a water-repellent layer having a smooth surface.
In view of this, it is an object of the present invention to provide a water-repellent layer 12 having a high bonding capability to a catalyst layer and a smooth surface without being affected by the surface condition of a gas diffusion layer 11. According to the present invention, the interstices that are created at the interface between the water-repellent layer 12 and the catalyst layer 13 can be reduced. Thereby, it is possible to reduce the contact resistance of the interface between the water-repellent layer 12 and the catalyst layer 13, to reduce the occurrence of flooding phenomenon, and consequently to provide a fuel cell electrode having an excellent power generation capability.